Integrated circuits are made up of literally millions of active devices such as transistors and capacitors. These devices are initially isolated from each other, and are later interconnected to form functional circuits. Typical interconnect structures include lateral interconnections, such as metal lines (wirings), and vertical interconnections, such as vias and contacts.
On top of the interconnect structures, connector structures are formed, and include bond pads or metal bumps formed and exposed on the surface of the respective chip. Electrical connections are made through the bond pads/metal bumps to connect the chip to a package substrate or another die. The electrical connections may be made through wire bonding or flip-chip bonding.
One type of the connector structures includes an aluminum pad, which is electrically connected to the respective underlying interconnect structure. A passivation layer and a polymer layer are formed, with portions of the passivation layer and the polymer layer covering edge portions of the aluminum pad. An Under-Bump Metallurgy (UBM) is formed to extend into the opening in the passivation layer and the polymer layer. A copper pillar and a solder cap may be formed on the UBM.